In refinery and petrochemical plants, gases are produced which, after processing in suitable blowdown recovery systems to retrieve condensates, are considered to be waste gases. These wastes usually represent a heterogeneous mixture of gases as a result of their having originated from a variety of sources such as hydrocarbon vapors from various leaks, or from venting unsafe operating pressures in process units during scheduled shutdowns and startups, or from certain plant failures which would cause sudden venting of gases. These large volumes of hydrocarbon gases produced in refinery plants are generally used as fuel or for raw material for further processing; however, sizeable quantities must be considered waste, and, after going through scubbers and knockout drums to gather condensates, it must be discarded as useless.
One common method used to dispense with waste gases includes use of vertical elevated flare stacks through which the gases are vented to atmosphere and ignited at the top by suitable pilot light means to produce burning in a smokeless flare. These flare stacks do not burn continuously but rather only as the upward flow of waste gases demands, controlled by suitable instrumentation governing ignitor means and perhaps steam-injection means located near the top of the stack.
Immediately after the flare may be caused to go out, gases within the flare stack system begin to cool down. Considering Charles' Law for gases at constant pressure (here open to atmosphere), V1/V2 = T1/T2, where V1 and V2 are the volumes of gas at absolute temperatures T1 and T2 respectively. If V1 and T1 represent volume and temperature respectively of a quantity of gas within a system, and V2 and T2 represent volume and temperature respectively of the same quantity at a lower temperature, from the rearranged equation V2 = V1(T2/T1) it can be seen the volume of gas varies directly as the ratio of temperatures on the absolute scale. For example, if 1,000 cubic feet of gas within a flare stack system is at 260.degree. C (533.degree. K) and cools to 16.degree. C (289.degree. K), its volume is reduced to 1000 .times. (289/533) or 542 cubic feet.
Cooling of gases in the flare stack system and the resulting reduction in volume following burning allows air to be drawn into the flare stack opening and would therein mix and cause an explosive condition.
Types of flare stack assemblies for limiting the entry of air into a flare stack are disclosed in the following patents: U.S. Pat. No. 3,055,417, granted Sept. 25, 1962, and U.S. Pat. No. 3,289,729, granted Dec. 6, 1966.
To prevent dangerous explosive conditions within a flare stack system, air must be kept out during non-burning periods. The most common means for so doing includes the introduction near the lower end of the flare stack of commonly available purge gas such as methane (natural gas) at rates sufficient to maintain a slow upward flow within the stack and thus prevent downward flow of air. Of course, their use of natural gas is, except for the safety benefits, an energy waste as it escapes into atmosphere. Because the stacks are typically large in diameter, and to insure an always upward flow, however, slow, the amounts of natural gas used for this purpose are not insignifcant when viewed ecologically during this time of concern for energy conservation.